Back to EveryPatent.com
United States Patent |
6,117,400
|
Naka
,   et al.
|
September 12, 2000
|
HC emission control member for exhaust gas
Abstract
An HC emission control member includes an HC adsorbing layer on a carrier
and an HC oxidizing layer on the HC adsorbing layer. The hydrophobic
property of the HC adsorbing layer is enhanced, so that satisfactory
adsorption and desorption of HC could be obtained.
Inventors:
|
Naka; Takahiro (Wako, JP);
Endo; Tetsuo (Wako, JP);
Shimizu; Haruhiko (Wako, JP);
Kikuchi; Shinichi (Wako, JP);
Wakabayashi; Mitsuo (Wako, JP)
|
Assignee:
|
Honda Giken Kogyo Kabushiki Kaisha (Tokyo, JP)
|
Appl. No.:
|
041102 |
Filed:
|
March 12, 1998 |
Foreign Application Priority Data
Current U.S. Class: |
422/180; 422/171; 422/177; 502/61; 502/64; 502/73; 502/235; 502/407 |
Intern'l Class: |
B01D 053/34; B01J 020/02 |
Field of Search: |
422/171,177,180
502/61-63,64,71,232-235,355,407,73
|
References Cited
U.S. Patent Documents
5125231 | Jun., 1992 | Patil et al. | 422/171.
|
5306684 | Apr., 1994 | Itoh et al. | 502/61.
|
5447694 | Sep., 1995 | Swaroop et al. | 422/171.
|
5525307 | Jun., 1996 | Yasaki et al. | 422/171.
|
5741948 | Apr., 1998 | Kirishiki et al. | 502/71.
|
Foreign Patent Documents |
2-56247 | Feb., 1990 | JP.
| |
Primary Examiner: Tran; Hien
Attorney, Agent or Firm: Arent Fox Kintner Plotkin & Kahn, PLLC
Claims
We claim:
1. An HC emission control member for an exhaust gas, comprising an HC
adsorbing layer on a carrier and an HC oxidizing layer on said HC
adsorbing layer wherein said HC adsorbing layer is made of a
metallo-silicate having an Al content of .ltoreq.0.05% by weight.
2. The HC emission control member according to claim 1, wherein said
metallo-silicate is an MFI metallo-silicate comprising at least one of Ga
and In as a skeleton forming element.
3. The HC emission control member according to claim 2, wherein said
metallo-silicate comprises Ga as a skeleton forming element.
4. The HC emission control members according to claim 2, wherein said
metallo-silicate comprises In as a skeleton forming element.
5. The HC emission control member according to claim 1, wherein said
carrier comprises a honeycomb made of cordierite.
6. The HC emission control member according to claim 1, wherein said HC
oxidizing layer comprises at least one of Pd, Pt and Rh as a catalyst
element.
7. The HC emission control member according to claim 2, wherein the MFI
metallo-silicate comprises 30% by weight of SiO.sub.2, 0.4% by weight of
Na.sub.2 O, and gallium chloride, with a purity of 99.999%.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an HC (hydrocarbon) emission control
member for an exhaust gas, which is used in an exhaust system in an
automobile or the like, and particularly, to an improvement in an HC
emission control member for an exhaust gas comprising an HC adsorbing
layer on a carrier and an HC oxidizing layer on the HC adsorbing layer.
2. Description of Prior Art
There are conventionally known exhaust emission control catalysts of
various configurations used in automobile exhaust systems. However, a
common exhaust emission control catalyst is able to purify an exhaust gas
by oxidation only if the exhaust gas is at a high temperature, e.g., about
180.degree. C. or more and hence, the catalyst has an extremely low
ability to purify an exhaust gas, which includes hydrocarbon (HC) at a
high concentration, having a low temperature immediately after the start
of an engine.
An HC emission control member, which is conventionally known, comprises an
HC adsorbing layer made of an aluminosilicate (zeolite) to catch HC in an
exhaust gas having a low temperature (for example, see Japanese Patent
Application Laid-open No. 2-56247).
However, the aluminosilicate adsorbs water along with HC in the exhaust gas
and the water adsorption reduces the HC adsorbing ability of the
aluminosilicate. When the aluminosilicate desorbs HC, the adsorbed water
is also desorbed. For this reason, it is difficult to raise the
temperature of the HC oxidizing layer to a temperature enough to activate
the HC oxidizing layer due to the heat of vaporization produced by water
evaporation. As a result, the conventional HC emission control member
could not satisfactorily adsorb and desorb HC.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an HC emission control
member of the above-described type, wherein the hydrophobic property of
the HC adsorbing layer can be enhanced, so that satisfactory adsorption
and desorption of HC could be obtained.
To achieve the above object, according to the present invention, there is
provided an HC emission control member for an exhaust gas, which comprises
an HC adsorbing layer on a carrier, and an HC oxidizing layer on the HC
adsorbing layer, wherein the HC adsorbing layer is made of a
metallo-silicate having an Al content of .ltoreq.0.05% by weight.
Being more hydrophobic due to the low content of Al, the metallo-silicate
has a low water-adsorbing property. Thus, the HC adsorbing layer
preferentially and sufficiently adsorbs HC, so that sufficient physical
adsorption is produced. On the other hand, the adsorbed HC is desorbed
with an increase in temperature of the exhaust gas, and when the HC is
desorbed, the amount of water desorbed is small. Therefore, there is no
inhibition of temperature increases in the HC oxidizing layer. As a
result, there will be satisfactory chemical conversion of the desorbed HC.
However, if the Al content is higher than 0.05% by weight, the hydrophobic
property of the metallo-silicate is reduced. It is desirable for enhancing
the hydrophobic property of the metallo-silicate that the Al content is
zero, but in the preparation of the metallo-silicate, it is difficult to
suppress the Al content to zero because of impurities in the starting
material. Therefore, a lower limit of the Al content is a value
assymptotic to zero.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view of an essential portion of an HC emission
control member.
FIG. 2 is a graph illustrating the relationship between the content of Al
and the water adsorption rate.
FIG. 3 is a graph illustrating the first example of the result of a TPD
analysis.
FIG. 4 is a graph illustrating the second example of the result of the TPD
analysis.
FIG. 5 is a graph illustrating the third example of the result of the TPD
analysis.
FIG. 6 is a graph illustrating the relationship between the gas temperature
and the HC emission control rate.
FIG. 7 is a graph illustrating the relationship between the time of
operation of an engine, the HC emission control rate and the temperature
of an HC oxidizing layer.
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 1, an HC emission control member 1 is comprised of a cordierite
honeycomb 2 as a carrier, and an HC converting laminate 4 provided on an
inner surface of each of cells 3 of the honeycomb 2. The laminate 4
includes an HC adsorbing layer 5 on the inner surface of the cell 3, and a
porous HC oxidizing layer 6 on an inner surface of the HC adsorbing layer
5.
The HC adsorbing layer 5 is made of a metallo-silicate having an Al content
of .ltoreq.0.05% by weight. The metallo-silicate may be an MFI type
metallo-silicate comprising at least one of Ga and In as a skeleton
forming element.
The HC oxidizing layer 6 comprises at least one of Pd, Pt and Rh as a
catalyst element, and may comprise an oxide of Ce, Zr, La, Ba or the like
as a co-catalyst.
The reason why the HC adsorbing layer 5 is situated beneath the inner
surface of the carrier is that the rise in temperature of the HC adsorbing
layer 5 caused by the exhaust gas can be inhibited so that the HC
adsorbing layer 5 can exhibit sufficient HC adsorbing abilities. On the
other hand, the reason why the HC oxidizing layer 6 is situated on the
inner surface of the carrier is that the rise in temperature of the HC
oxidizing layer 6 caused by the exhaust gas can be facilitated so that the
HC oxidizing layer 6 can exhibit sufficient HC oxidizing abilities.
Examples will be described below.
I-1. Production of MFI gallium-silicate
(1) 0.4 kg Of TPA-Br (tetrapropylammonium bromide, a template agent) and 5
kg of pure water were added to 6.5 kg of commercially available colloidal
silica (including 30% by weight of SiO.sub.2 and 0.4% by weight of
Na.sub.2 O) to prepare 11.9 kg of a first starting material.
(2) 0.3 kg of sodium hydroxide and 8 kg of pure water were added to 0.08 kg
of gallium chloride (GaCl.sub.3 with a purity of 99.999%) to prepare 8.38
kg of a second starting material.
(3) The first starting material was placed into a container made of a
stainless steel, and the second starting material was gradually added to
the first starting material, while the first starting material was being
stirred.
(4) The mixture of the first and second starting materials was stirred for
30 minutes to provide a gallium silicate alkali gel which was uniform all
over. The composition of the alkali gel was as follows: Molar ratio of
SiO.sub.2 /Ga.sub.2 O.sub.3 =750; molar ratio of Na.sub.2 O/SiO.sub.2
=0.133; molar ratio of H.sub.2 O/Na.sub.2 O=226; and molar ratio of
TPA-Br/SiO.sub.2 =0.05.
(5) The alkali gel was thrown into an autoclave and maintained at
170.degree. C. for 24 hours with stirring for crystallization, thereby
providing a slurry of crystals.
(6) The slurry of crystals was subjected to a solid-liquid separation
treatment to provide a solid component, which was washed and then filtered
to provide a cake.
(7) The cake was dried at 110.degree. C. for 24 hours and then calcined at
550.degree. C. for 12 hours using an electric oven. Thereafter, the
calcined material was pulverized to provide about 1.3 kg of a powdery MFI
gallium silicate.
The content of Ga in the MFI gallium silicate was equal to 0.4% by weight,
and the content of Al was equal to 0.04% by weight. The aluminum is
believed to be incorporated in the commercially available colloidal
silica.
I-2. Production of MFI Indium Silicate
About 1.3 kg of a powdery MFI indium silicate was produced in the same
manner as in Example I-1, except that indium chloride (InCl.sub.3 with a
purity of 99.999%) was used in place of gallium chloride.
In this case, the composition of the indium silicate alkali gel was as
follows: Molar ratio of SiO.sub.2 /In.sub.2 O.sub.3 =750; molar ratio of
Na.sub.2 O/SiO.sub.2 =0.133; molar ratio of H.sub.2 O/Na.sub.2 O=226; and
molar ratio of TPA-Br/SiO.sub.2 =0.05. The content of In in the MFI indium
silicate was equal to 0.6% by weight, and the content of Al was equal to
0.04% by weight.
I-3. Production of MFI Aluminosilicate
About 1.3 kg of a powdery MFI aluminosilicate (ZSM-5 zeolite) was produced
in the same manner as in Example I-1, except that sodium aluminate
(comprising 52.7% by weight of Al.sub.2 O.sub.3 and 41.9% by weight of
Na.sub.2 O) was used in place of gallium chloride.
In this case, the composition of the aluminosilicate alkali gel was as
follows: Molar ratio of SiO.sub.2 /Al.sub.2 O.sub.3 =750; molar ratio of
Na.sub.2 O/SiO.sub.2 =0.133; molar ratio of H.sub.2 O/Na.sub.2 O=226; and
molar ratio of TPA-Br/SiO.sub.2 =0.05. The content Al in the MFI
aluminosilicate was equal to 0.2% by weight.
I-4. Content of Al
Various MFI gallium silicates having different Al contents were produced in
the same manner as in the production of the MFI gallium silicate described
in Example I-1, except that alumina was added in varied amounts to the
alkali gel.
Each of the MFI gallium silicates was left to stand for 70 hours in an
atmosphere having a temperature of 40.degree. C. and a humidity of 90%.
Then the relationship between the content of Al in the MFI gallium
silicate and the water absorbing rate was examined, thereby providing
results shown in FIG. 2. It can be seen from FIG. 2 that if the content of
Al is .ltoreq.0.05% by weight, the hydrophobic property of the MFI gallium
silicate is largely enhanced.
I-5. HC Desorbing Characteristic
The MFI gallium silicate, the MFI indium silicate and the MFI
aluminosilicate were heated at 500.degree. C. for 5 hours in air.
The following two atmospheres were prepared: a first atmosphere consisting
of 100 ppm of C.sub.7 H.sub.14 and the balance N.sub.2 and containing no
water and a second atmosphere consisting of 100 ppm of C.sub.7 H.sub.14,
10% by volume of H.sub.2 and the balance N.sub.2 and water. The MFI
gallium silicate, the MFI type indium silicate and the MFI aluminosilicate
were left to stand for 20 hours in the first atmosphere, thereby
permitting C.sub.7 H.sub.14 to be adsorbed to the silicates in a
water-free condition. At the same time, the three silicates were likewise
left to stand for 20 hours in the second atmosphere, thereby permitting
C.sub.7 H.sub.14 to be adsorbed to the silicates in a water-containing
condition.
Thereafter, each of the silicates was subjected to a TPD (Thermo Programmed
Detector) analysis at a He flow rate of 29 ml/min and a rate of silicate
temperature increase equal to 5.degree. C./min to examine the HC desorbing
characteristic, thereby providing results shown in FIGS. 3 to 5.
As apparent from FIGS. 3 and 4, in the case of the MFI gallium silicate and
the MFI indium silicate, the HC desorbing characteristic after HC
adsorption carried out in the water-containing condition is substantially
equivalent to and not varied from that after HC adsorption in the
water-free condition, because these silicates have Al contents equal to or
lower than 0.05% by weight and are quite hydrophobic. However, in the case
of the MFI aluminosilicate, a relatively large difference is produced
between the HC desorbing characteristics due to the presence or absence of
water as shown in FIG. 5. This is due to the fact that the amount of HC
adsorbed in the water-containing condition is small, as compared with that
in the water-free condition, because the MFI aluminosilicate is less
hydrophobic.
I-6. Relationship Between the HC Oxidizing Layer and Water
[A] Fabrication of HC Oxidizing Layer
(1) .gamma.-Alumina, palladium nitrate, SiO.sub.2 sol and water were
metered to provide a ratio of 10:3.3:0.7:30 by weight, and then mixed
together. Then a predetermined amount of alumina balls were incorporated
into the resulting mixture and, thereafter, the mixture was mixed and
pulverized in a ball mill to provide a palladium slurry.
(2) A honeycomb made of cordierite was immersed into the palladium slurry,
whereby the palladium slurry was deposited on inner surfaces of cells of
the honeycomb.
(3) The palladium slurry was dried at room temperature and then, the
honeycomb was placed into an electric oven, where it was calcined in air
for one hour each at temperatures of 100.degree. C., 200.degree. C.,
300.degree. C. and 600.degree. C. to provide an HC oxidizing layer 6
including Pd. In this case, the amount of Pd contained was 7.7 g/liter.
[B] HC Converting Ability of HC Oxidizing Layer
A test gas was prepared with varied amounts of water of 0, 10 and 20% by
volume, as shown in Table 1.
TABLE 1
______________________________________
Test Gas
Constituent Concentration
______________________________________
O.sub.2 0.5% by volume
CO.sub.2 14% by volume
CO 0.5% by volume
C.sub.3 H.sub.6
1200 ppmc
H.sub.2 0.17% by volume
NO 0.05% by volume
H.sub.2 O Varied Amounts
(of 0, 10 or 20% by volume)
N.sub.2 Balance
______________________________________
The test gas was allowed to flow through the honeycomb at a space velocity
S.V. of 50,000h.sup.-1, while at the same time, the temperature of the
test gas was increased from ambient temperature to 300.degree. C. The HC
emission control rate during this time was measured, thereby providing
results shown in FIG. 6.
As apparent from FIG. 6, if the amount of water in the test gas is
increased, the HC emission control rate at the same gas temperature of
about 175.degree. C. to about 250.degree. C. is decreased. From this fact,
it can be seen that the activity of the HC oxidizing layer is influenced
by water and is higher with a smaller amount of water.
II-1. Production of HC Emission Control Member
[A] Example
(1) An MFI gallium silicate, SiO.sub.2 sol and water were metered to
provide a ratio of 10:1:10 by weight and mixed together. Then a
predetermined amount of alumina balls were incorporated into the resulting
mixture and, thereafter, the mixture was mixed and pulverized in a ball
mill to provide a silicate slurry.
(2) A honeycomb 2 made of cordierite was immersed into the silicate slurry,
whereby the silicate slurry was deposited onto inner surfaces of cells 3
of the honeycomb.
(3) The silicate slurry was dried at room temperature. Then, the honeycomb
2 was placed into an electric oven, where it was calcined in air for one
hour each at temperatures of 100.degree. C., 200.degree. C., 300.degree.
C. and 600.degree. C. to provide an HC adsorbing layer 5. In this case,
the amount of HC adsorbing layer 5 obtained was 110 g/liter.
(4) The honeycomb 2 having the HC adsorbing layer 5 was immersed into a
palladium slurry similar to that described in Example 1-6, whereby the
palladium slurry was deposited onto the inner surface of the HC adsorbing
layer 5 within each cell of the honeycomb.
(5) The palladium slurry was dried at room temperature and then the
honeycomb was placed into an electric oven, where it was calcined in air
for one hour each at temperatures of 100.degree. C., 200.degree. C.,
300.degree. C. and 600.degree. C. to provide an HC oxidizing layer 6
including Pd. In this case, the amount of HC oxidizing layer 6 obtained
was 57 g/liter.
[B] Comparative Example
An HC emission control member 1 was produced by carrying out an operation
similar to that in Example [A], except that MFI aluminosilicate was used
in place of the MFI gallium silicate. In this case, the amount of HC
adsorbing layer 5 was 110 g/liter, and the amount of HC oxidizing layer 6
was 60 g/liter.
II-2. HC Emission Control Ability
The HC emission control member according to Example [A] was incorporated
into an exhaust system of an engine (of a displacement of 2200 cc) to
measure the HC emission control rate at the initial stage of the start of
the engine. A similar measurement was also carried out using the HC
emission control member 1 produced according to the comparative example,
Example [B].
FIG. 7 shows the relationship between the operation duration of the engine,
the HC emission control rate and the temperature of the HC oxidizing
layer. In FIG. 7, a time point when the engine had been operated for 12
seconds was a transition point between physical adsorption and chemical
conversion, and at this time point, the HC emission control rates of the
HC emission control members according to Example [A] and the comparative
example were lowest.
The lower limit value of the HC emission control rate of the HC emission
control member according to Example [A] was about 60%. Thereafter, the HC
emission control rate was rapidly increased with a rise in the temperature
of the HC oxidizing layer 6. When the engine had been operated for 40
seconds, an HC emission control rate of 90% or more was obtained.
The lower limit value of the HC emission control rate of the HC emission
control member according to the comparative example was as low as about
35%. Thereafter, the HC emission control rate showed a smaller increase,
compared with the HC emission control member according to Example [A],
with a rise in the temperature of the HC oxidizing layer. When the engine
had been operated for 40 seconds, the HC emission control rate was only
about 80%.
The reason why a difference was produced between HC emission control rates
at the initial stage of the start of the engine in the HC emission control
members according to Example [A] and the comparative example is that the
HC adsorbing layer 5 in Example [A] was more hydrophobic than the HC
adsorbing layer 5 in the comparative example.
In the MFI gallium silicate, the Ga content therein should be in a range of
0.01% by weight to .ltoreq.0.5% by weight. If the Ga content is lower than
0.01% by weight, the crystallizability is poor. On the other hand, if the
Ga content is >0.5% by weight, the molar ratio of SiO.sub.2 /Ga.sub.2
O.sub.3 is decreased and hence, the selective adsorption performance for
HC is reduced.
In the MFI indium silicate, the In content therein should be in a range of
0.01% by weight to .ltoreq.0.6% by weight. If the In content is lower than
0.01% by weight, the degree of crystallization is low, resulting in an
unstable structure. On the other hand, if the In content is >0.6% by
weight, the molar ratio of SiO.sub.2 /In.sub.2 O.sub.3 is decreased and
hence, the selective adsorption performance for HC is reduced.
Finally, it should be noted that the MFI metallo-silicate may contain both
Ga and In as skeleton forming elements.
Top